Method and apparatus for detecting command wire utilized to detonate an improvised explosive device (ied)

ABSTRACT

A method and apparatus is devised for detecting command wires utilized to detonate an improvised explosive device or other objects of interest in which frequency-scanned RF in the HF region of the electromagnetic spectrum is projected out across a given area and returns are detected and converted into image data in which phase, amplitude, range and frequency associated with the incoming data is correlated with frequency-dependent range templates to determine the existence of, the range of and the direction of command wires or other objects of interest.

FIELD OF THE INVENTION

This invention relates to a system for detecting command wires that areused to activate explosives and more particularly to resonance sensingand the utilization of range templates to detect the existence of andthe range to command wires utilized for the firing of ImprovisedExplosive Devices.

BACKGROUND OF THE INVENTION

Deployed forces face an increasing number of threats from theiradversaries. Of chief concern is the Improvised Explosive Device (IED).IEDs are generally hidden near or underneath roadways and remotelydetonated by an operator stationed nearby. While remote detonationmethods include radio frequency, infrared and hydraulic actuators, byfar the more difficult to detect is the use of the common command wire.

The command wires are typically embedded in the ground where they canneither be seen or readily detected. Thus, convoys have routinely beenattacked by improvised explosive devices which are buried andcamouflaged so that neither the explosive nor the command wire isreadily observable.

Since RF detonating devices can be sensed by detecting RF energy fromthe devices, these RF devices can be countermeasured because theirexistence is ascertainable. However, the use of wires to detonateexplosives from for instance distances of 100 to 500 feet provide astealth component to IED detonation because the wires themselves arevery thin and easily hidden or buried.

Thus, remote detection of command wires embedded in towns and citiesabove and below ground is a daunting task. Since microwave UHF signalsand VHF signals cannot effectively penetrate into the infrastructures ofthe city or below ground, one has to investigate what near fieldapproaches can be applied that operate in a frequency range which willin fact penetrate local objects in the ground.

SUMMARY OF INVENTION

The existence of and the length of as well as the range of a controlwire is detected through a resonance technique in which frequency sweptenergy in the HF band, typically from 1.5 to 20 megahertz, is projectedtowards a given area and radiation returned from the area is analyzed.The transmitter used in one embodiment steps its output from 1.5megahertz to 20 megahertz in 50 kilohertz frequency bins or steps. Thereturns from the exposed area are monitored using a near field scanningsensor operating in the HF band in which in its simplest implementationinvolves a single high Q switchable antenna which acts as anelectromagnetic probe, for instance mounted on a ground-based vehicle orunmanned aerial vehicle. As the vehicle travels along the road, theprobe collects mutual impedance data between it and the surroundinginfrastructure. This data is converted to an image in which amplitudeand phase are correlated with range to provide a range profile for eachof the stepped frequencies.

It has been found that there is a near field region about 300 meters indiameter where fields are highly inhomogeneous. This inhomogeneity givesrise to unique features associated with the geometries of the objects ofinterest. One object of interest is a command wire having acharacteristic resonance. The returns from the wire are analyzed interms of frequency, phase and range, with range-correlated images beinggenerated for each, frequency bin.

The area under surveillance is previously mapped to providefrequency-dependent range-correlated image templates (range templates)which are arranged in increased frequency steps. The range-correlatedimages generated during the sensing are correlated against rangetemplates, each one of the range templates being associated with a givenlength wire and a given orientation of the wire relative to forinstance, direction of travel of the vehicle carrying the antenna orantennas.

Upon correlation between the images formed from the returns and therange templates stored as a result of the survey, the stored templatemost closely matching the image formed from the returns is returned todescribe both the existence of a wire, its length, and its range.

One way to describe the subject invention is that it is similar tosynthetic aperture radars in which images of the infrastructure aregathered as a vehicle proceeds. There is a near field region about 300meters in diameter where the fields are highly inhomogeneous and 2-Dimages with range as one axis and frequency as the other are formed asthe vehicle proceeds. The image contains pixels that constitute complexaggregate mutual impedances, or in this case having frequency, phase,amplitude and range attributes.

In one embodiment an S-parameter receiver, in essence a networkanalyzer, detects the incoming signal and, generates a complex,reflection coefficient S-11 value that is coupled to a frequency/rangeimage generator, which is in turn coupled to an image processor. Thisimage processor generates an image in which range is mapped with respectto frequency, phase and amplitude of the incoming signal. The amplitudeand phase responses with frequency are combined into a range-dependantimage, with the range dependant image correlated with surveyed andstored templates, each one designating a wire of a predetermined lengthand a predetermined orientation for a predetermined range. Correlationof the two images provides the identity of the stored range templatewhich most closely correlates to the sensed data, thereby to identify 1)the existence of a wire, 2) the length of the wire, and 3) the range tothe wire.

In one embodiment, 370×150 pixel template images of objects of interestare generated and incorporated into the system. Incoming data iscontinuously correlated against the library of objects of interest inreal time. While the subject invention has been described in terms ofcommand wires, other objects of interest having characteristic complexpermeability and dielectric constants are within the scope of thisinvention.

In one embodiment, a Greens function-like template is used to “map” thecity or town. Clutter rejection is possible by comparing or subtractingpreviously taken data along the same route. The above is particularlyattractive since changes in the environment, provide important clues asto the current situation.

Experiments have shown that with an output power of 1 milliwatt and acoherent integration time of 750 milliseconds, a 150 foot piece of wirelying on the ground at a range of 100 meters with mediumurban/atmospheric daytime noise is detectable with a signal-to-noiseratio of 50 dB at 3.0 MHz. Although there is copious energy available,the effect of clutter from surrounding infrastructure can presentproblems. If however the images of interest are very different from theclutter, then the correlation pattern recognition is effective todetermine an object of interest, namely the existence of a wire. Notethat finite difference or gradient sampling is well suited to thesubject invention since the near field used to scan the object ofinterest is rapidly changing.

In one embodiment, in order to detect the resonance associated with thecommend wire the frequency is stepped from 1.5 megahertz in 40 steps.With each step the antenna used for the transmission or receipt of thereturn signals is appropriately tuned.

A single antenna is used in one embodiment for both transmitting andreceiving, requiring an S-11 receiver.

When utilizing a second antenna as a receive antenna, the receiver is anS-21 mode receiver, with the subject system looking at the transferfunction between the two antennas. Here the transfer function changes inthe presence of a wire such that the wire is detectable, even though thewire may be 100 feet ahead of the two antennas.

Thus, with a single antenna and the receiver operating in the S-11 mode,amplitude, phase, frequency and range are measured in terms of thesignal returned at the sole antenna.

In the S-21 mode utilizing a transmit antenna and a receive antenna,what is measured is the transfer function between the transmit antennaand the receive antenna.

If for instance the half-wave resonance wavelength is 500 feet, onetypically has an antenna length for a half-wave antenna of 160 meters orabout 250 feet. With a 500 foot long length of wire, one would see aresponse at about 900 kilohertz. If there is a half-wave resonance at900 kilohertz, there will also be a three half-wave resonance at 2700kilohertz, etc. Thus when one is sweeping through the frequencies ineither the S-11 or S-21 mode, one will see resonant peaks at onehalf-wave length, three half-wave lengths, or five half-wave lengths.One might also see peaks at one wavelength as well as both odd and evenharmonics depending on how the wire is configured.

It is noted that there will be peaks in amplitude at given ranges. Therewill also be phase peaks in terms of the phase function for thedifference in phase between the outgoing signal and the reflectedsignal. Note that the progression of the phase will be different fordifferent frequencies, different resonances and different ranges. It hasthus been found that phase is a function of range for differentfrequencies, enabling one to template out the effect.

Note also that with respect to amplitude, amplitude will increase incertain manner characteristic of attenuation along the ground so thatanother way of obtaining range is to measure amplitude.

What has been found is that for both amplitude and phase there is acharacteristic that is a function of range. Thus, the range informationin both the amplitude and phase domains can be used simultaneously toobtain range.

Moreover when using the S-21 receiver and two antennas, one candetermine to which side of a road the wire extends. By appropriatephasing of the two antennas one can provide null and non-nulldirections. Thus, if there is a return from a non-null direction it canbe ascertained to which side of the line of travel the wire is running.

As will be appreciated, in the S-11 mode one is looking to analyze thecomplex reflection coefficient. The complex reflection coefficient is aresult of looking at the reflected power from a load or from an antenna.As the antenna approaches the command wire the reflected power starts tochange and by monitoring the reflected power one can sense the wire in arather sensitive manner.

Thus in the S-11 mode, one is looking for changes in reflected power interms of phase and amplitude over frequency correlated with range.

In the S-21 mode, since one is looking for changes in the energy that ispicked up versus that which is transmitted, the changes are measured interms of the transfer function between the two antennas.

In one embodiment where the lowest operating frequency is 1.5 megahertz,the antennas can take the form of large coils which are tapped fortuning purposes, with the taps being set by relays or electronicswitches that are tracking the transmit frequency.

In summary, a method and apparatus is devised for detecting commandwires utilized to detonate an improvised explosive device or otherobjects of interest in which frequency-scanned RF in the HF region ofthe electromagnetic spectrum is projected out across a given area andreturns are detected and converted into image data in which phase,amplitude, range and frequency associated with the incoming data iscorrelated with frequency-dependent range templates to determine theexistence of, the range of and the direction of command wires or otherobjects of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description, in conjunctionwith the Drawings, of which:

FIG. 1 is a diagrammatic illustration of the implanting of an IED in aroadway, with the IED detonated using a buried command wire;

FIG. 2 is a block diagram of a scanning near-field electromagnetic probein which tunable high Q electromagnetic probes are coupled to a receiverthat outputs a signal applied to a frequency/range image generator, theoutputs of which are compared in an image processor to stored images toderive the correlation between the data associated with the measurementand the stored images, thus to provide the identity of, the existence ofand the range of an object of interest;

FIG. 3 is a diagrammatic illustration of the subject system in whichtransmitted energy is received and an S-parameter receiver is employedto output amplitude, phase and frequency of the returned signal graphedagainst range in which wire resonance is determined both by amplitude,phase and frequency with respect to range, with the image compared orcorrelated with survey templates to ascertain the existence of a wire,and the length of and range to the wire;

FIG. 4 is a graph of amplitude versus frequency indicating wireresonance points at the one half-wave resonance, the three half-waveresonance and the five half-wave resonance frequencies;

FIG. 5 is a graph of phase versus frequency showing a one half-waveinflection in the phase plot indicating the presence of a wire;

FIG. 6 is a diagrammatic illustration of the utilization of a phasedantenna array on a vehicle for left/right detection of the direction ofa wire extending from the middle of a roadway in which a non-nullresponse along a line to the right of the roadway indicates thedirection in which the command wire is running;

FIG. 7 is a block diagram of the utilization of amplitude, phase,frequency and range matched with the same parameters forsurvey-generated range templates to provide for the detection of a wire,its length and range;

FIG. 8 is an image generated from wire reflections in which frequency isplotted against range, with the density of the pixels indicatingamplitude showing the resonant response at 3 MHz, 9 MHz and 15 MHzindicating resonance characteristics associated with the existence of a150 foot wire on the ground at 100 meters in front of a vehicle;

FIG. 9 is an image generated from wire reflections indicating a phaseinflection response at 3 MHz corresponding to the phase response to thepresence of a 150 foot wire on the ground 100 meters in front of avehicle;

FIG. 10 is an image of the response of the subject system to anautomobile, showing the modeled amplitude response of an automobile at100 meters in front of a vehicle;

FIG. 11 is an image of the combined 150 foot wire, automobile and powerline response, illustrating amplitude peaks at the one-half, three-halfand five-half wavelengths, with the automotive response indicated as asmeared-out area to the bottom right of the image and with power lineresponses having a different periodicity;

FIG. 12 is a graph of frequency versus amplitude for a 150 foot wire ata predetermined range showing a one-half wave resonance at 3.2 MHz and asecond three-half wavelength resonance at 9 MHz; and,

FIG. 13 is a graph of amplitude versus frequency for a 300 foot wire atthe predetermined range of FIG. 12, indicating a three-half wavelengthresonance at approximately 5.5 MHz.

DETAILED DESCRIPTION

Referring now to FIG. 1, an improvised explosive device 10 is placedbeneath a road 12 and is activated by a command wire 14 coupled to adetonator 16 in the form of a conventional plunger assembly.

IED 10 is placed along the direction of travel 18 of a vehicle 20, withthe purpose of the stealth IED and command wire being to surprise thevehicle and then blow it up.

It is the purpose of the subject invention to detect such stealthdevices to permit countermeasuring.

In order to provide for early enough warning of the existence of acommand wire in one embodiment, as illustrated in FIG. 2, a pair oftunable high Q electromagnetic probes 30 and 32, in one embodiment inthe form of HF coil antennas, are coupled to an S-parameter receiver 34,the output of which is the unitless complex reflection coefficient S-11,namely involving amplitude, phase and frequency. The heart of the Sparameter receiver is a network analyzer. Either S11 or S12 modes may beused. S11 requires just one probe while S12 requires two probes. Theprobes are switch tunable high Q antennas. The idea of the S11 mode isto sample the mutual impedance of the probe to objects in the nearfield. The idea of the S12 mode is to sample the transfer functionbetween the two probes in the presence of objects in the near field.

Critical to the sensitivity of the S parameter receiver is the use of aset of reference data at all the frequencies involved. In theinitialization phase, each time the receiver is turned on, the system ispositioned in a near field object free zone and S11 or S12 data is takenover all frequencies. This set of data is used as a reference such thatthe working output of the receiver is S11-S11 (ref) or S12-S12 (ref).Thus if the system is initialized and operation is started in the nearfield object free zone, the output of the receiver is zero. This type ofoperation gives equivalent performance to a receiver with a low noisefigure, dependent upon the accuracy of the above subtraction. When anA/D converter is used, the subtraction is done after the A/D converterso that the noise figure and dynamic range are limited by the number ofbits. An alternative mode of operation, useful when the system isinstalled on a moving vehicle, is the differential mode. Thedifferential mode subtracts sequential pairs of data sets. This modelooks for differences in S11 or S12 as the vehicle progresses. Thedifferential mode requires no absolute reference set.

In the subject invention amplitude, phase and frequency are correlatedwith range. This means that for each range an image can be generated,the pixels of which reflect amplitude, phase and frequency.

To this end the output of the S-parameter receiver is coupled to afrequency/range image generator 36 which provides an image comprised ofI, Q complex pixels in which for each frequency there is a range image,with the pixel intensities indicating amplitude and phase of the signalreturned from the scene.

The images are coupled to an image processor 38, in one embodimentprovided with the position of the probe from a GPS unit 40. The imagefrom the realtime data is compared or correlated with stored image dataat 42 comprising templates that are the result of a survey of the areain question. In one embodiment, the survey for detection of a commandwire includes amplitude, phase and frequency for various ranges ofvarious length wires in various orientations with respect to the path ofa vehicle.

The image processor output is correlated with the stored images atcorrelator 44, with a correlation being output at 46 to warn theoperator of the vehicle that a wire exists, its range and its direction.With this type of information countermeasures may be deployed to takeout the control wire or to take out the operator at the detonator at oneend of the wire.

Note that a control 48 controls the tuning of the high Q EM probes 30and 32, as well as the stepped frequency transmission of theelectromagnetic energy projected into the scene by the transmitterutilized.

Referring to FIG. 3, it can be seen that a transmitter 50 is frequencyswept by a variable frequency oscillator 52 to project energy out from atuned antenna 54 into the probe environment. The antenna is tuned asillustrated by 56 to be resonant at the requisite stepped frequencybins, thereby providing extremely good sensitivity across the steppedfrequency range.

GPS 40 is coupled to S-parameter receiver 34 which detects returns fromthe scene and outputs amplitude 60, phase 62 and frequency 66 that areutilized to generate the images described above.

As can be seen at graph 70, a half-wave phase resonance plot atfrequency f₁ is shown by dotted line 72, with the plotted phase definedby the phase of the outgoing signal compared to the phase of thereflected signal. It will be seen that the phase plot at half-waveresonance has the characteristic shown at 74 for a wire of a particularlength and orientation versus range.

Graph 70 also shows an amplitude plot 76 that has a particular shape forthe one-half wave resonance that is peculiar to a wire of a particularlength and orientation versus range.

As shown at 76′ for an off-resonance graph, the amplitude plot takes onquite a different configuration as does the phase plot 74′ indicating atleast in these two graphs that there is a substantial difference betweenthe one-half wavelength resonance response of the system and theoff-resonance response.

As seen, images 80 are generated for frequencies f₁, f₂, f₃ . . . f_(n)in which the images are composed of pixels having densitiescorresponding to a combination of phase and amplitude versus range foreach frequency. These images therefore characterize the response of thesensing system for the stepped frequency bins involved.

Survey templates 90 involve range templates 92 for the frequencies f₁,f₂, f₃ . . . f_(n) which are utilized to model the returns from a givenlength wire at a given orientation with respect to the line of travel ofthe vehicle. These templates are used in a correlation process carriedout by correlator 44, with the closest correlation defining that surveytemplate to which the incoming data most closely approximates. Thisidentifies the existence of a wire, the length of the wire and the rangeof the wire, as well as its direction when multiple phased antennas areused.

Referring to FIG. 4, the graph shows amplitude versus frequency for agiven length wire and shows that there are amplitude peaks at theone-half wave resonance, three-half wave resonance and five-half waveresonance for the particular wire. Thus, there are different resonancecharacteristics associated with the detected wire.

It will be seen that these resonances are equally spaced in a patternwhich is highly recognizable and at the very least specifies the lengthof the wire.

In terms of phase, and referring now to FIG. 5, the phase of theincoming signal versus the phase of the outgoing signal is plotted withrespect to frequency, with an inflection 100 occurring at the one-halfwavelength responses, here shown at 3 MHz.

From FIGS. 4 and 5 it will be appreciated that the resonancecharacteristic associated with a wire can be sensed both in amplitudeand phase which, for a given range are uniquely determinative of therange of the wire from the sensing head.

Referring to FIG. 6, if a vehicle 20 is provided with phased arrayantennas 100 and 102, assuming an antenna array phasing module 104 isemployed to steer the array, and assuming that there is a null which canbe steered across the scene where an IED 10 and command wire 14 exist,it will be appreciated that if there is a response at a non-nulldirection 106, it will mean that the wire exists to the right of roadway12, whereas if there is no response when the beam is directed in itsnull direction as illustrated at 108, then the right/left direction ofthe wire is determinable.

Referring to FIG. 10, the subject system can be characterized by amatched filter 110 utilized to correlate the detected data tosurvey-generated range templates, with the wire existence, length andrange being outputted as illustrated at 114.

The amplitude sensitivity of the subject system is, illustrated in FIG.8. Here in an image 120 in which frequency is graphed against rangeexhibits pixels having amplitudes 122 that appear to be regularly spacedalong the frequency axis. This regular spacing indicates the amplituderesonance characteristic of the returned signal in which the resonancesare highly recognizable.

Referring to FIG. 9, the phase response of the subject system is shown.Here, frequency is graphed against range, with the pixel densitiesindicating the phase angle. Note there is a recognizable phase response124 which lies within the half-wavelength band 126. While the phaserelationships in the three-half wavelength resonance band 128 or thefive-half wavelength resonance band 130 are not so highly recognizable,by correlation with the aforementioned templates recognizable patternsfor the phase-related resonance effects of the wire can be ascertained.Note that the darkening of the pixels in bands 128 and 130 indicatesphase inflections associated with higher resonances.

Referring to FIG. 10, image 140 indicates the modeled amplituderesonance of an automobile at 100 meters in front of a vehicle, in whichthe detected response is a smeared out increase in amplitude for thepixels shown in area 142.

Referring to FIG. 11, when one considers the amplitude response of awire, here illustrated at 150, one can see the regular amplituderesonances for a wire which have a different periodicity than forinstance power lines having resonance peaks 152.

Also noted is the increased density of the pixels representing thepresence of a automobile. These appear in the lower right hand side ofthe image as illustrated at 154.

It can therefore be seen that the subject system has a uniquelysensitive response for detecting resonances due to the presence of awire, given the correlation process in which range templates arecompared with realtime data.

Referring now to FIGS. 12 and 13, amplitude graphed against frequencyfor a 150 foot wire and a 300 foot wire. As can be seen from FIG. 12,graph 160 shows amplitude resonances for the 150 foot wire at 162 and164, corresponding to the half and three-half wave resonances for the150 foot wire. When detecting a 300 foot wire at the same range, asillustrated by graph 170 there is a one-half resonance peak at 172. Thisconfirms that the length of the wire determines the resonancecharacteristics and that the resonances are detectable by the subjecttechnique.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

1. A method for detecting the presence of a control wire used todetonate an explosive, comprising the steps of: generating a number ofrange templates, each for a different frequency and each containingphase and amplitude information pertaining to a number of wires of anumber of lengths in a number of orientations; transmitting steppedfrequent signals from a probe towards a predetermined area; detectingthe amplitude and phase of energy reflected back to the probe;generating an image of the returned energy from the probe for eachfrequency of the probe transmission, the image including a range axisand an orthogonal amplitude/phase set of axes for each of the steppedfrequencies; and, correlating the image formed at the each of thestepped frequencies to the stored range templates, with a highcorrelation to one of the surveyed templates indicated by the identityof the existence of the wire, the length of the wire and the range ofthe wire to the probe.
 2. The method of claim 1, wherein the energy isprojected to the area utilizes a transmitter having a variable frequencyoutput stepped in accordance with a predetermined number of frequencybins.
 3. The method of claim 2, wherein the swept frequency range is inthe HF frequency band.
 4. The method of claim 3, wherein the number ofsteps in the stepped frequency band are between 30 and
 50. 5. The methodof claim 1, wherein the transmitting step includes a transmitter coupledto a transmitting probe antenna and wherein returned signals arereceived at a separate probe receive antenna, and further including thestep of steering the two antennas so as to establish a swept nullposition and determining from the presence of returned energy at anon-null position the direction to the wire from the probe.
 6. Themethod of claim 1, wherein the phase measured is the phase angle betweenthe transmitted signal and the return signal.
 7. The method of claim 1,wherein the swept frequency range is between 1.5 to 20 MHz.
 8. Themethod of claim 7, wherein the stepped frequency bins are 50 kilohertzin width.
 9. Apparatus for the detection of an object in a surveyed areacomprising: a transmitter for transmitting a transmitted signalcontaining RF energy into said area in a frequency swept manner; anantenna for receiving returned signals from objects in the surveyedarea; an S-parameter receiver coupled to said antenna for outputting theamplitude and phase of a returned signal for a predetermined one of saidswept frequencies; a frequency/range image generator coupled to saidS-parameter receiver for generating for each of the stepped frequenciesan image having range on one axis and amplitude and phase on orthogonalaxes, such that the pixels of said image have intensities correspondingto the amplitude of the returned signal and the phase angle between thecorresponding returned signal and the transmitted signal; an imagestorage unit for storing range template images corresponding to theresonance response of the object at various frequencies and ranges fromsaid antenna; and, a correlator for correlating the images generated asa result of returned signals to said antenna and the stored images, saidcorrelator including a module for identifying that stored image thatcorresponds most closely to the image that is a result of the returnedsignal, the identity of the stored image indicating the presence of saidobject and the range of said object to said antenna.
 10. The apparatusof claim 9, wherein said object is a wire.
 11. The apparatus of claim10, wherein said stored images include images obtained from a survey ofwires of different orientations and lengths.
 12. The apparatus of claim9, and further including an additional antenna coupled to saidS-parameter receiver, and further including a phasing unit for phasingsaid two antennas such that a null direction is established by saidphasing unit with respect to a centerline between said two antennas. 13.The apparatus of claim 12, and further including a detector fordetecting to which side of said centerline said returns are coming from,thereby to establish a right/left direction of said returned signals.14. Apparatus for detecting a command wire comprising: a transmitter fortransmitting stepped frequency signals to a surveyed area; and, a wiredetection module for detecting from returned signals the presence ofsaid wire by detecting the resonant frequency response of said wire tosaid transmitted signals.
 15. The apparatus of claim 14, wherein saidresonance response indicates the length of said wire.
 16. The apparatusof claim 14, wherein said resonance response indicates the range of saidwire to said wire detection module.
 17. The apparatus of claim 14,wherein said resonance response indicates the orientation of said wire.18. The apparatus of claim 14, wherein said resonance response isdetermined by said wire detection module generating an image from thereturned signal and correlating said image with stored image templates.19. The apparatus of claim 18, wherein stored image templates includerange templates.
 20. The apparatus of claim 19, wherein said templatesare generated, one each for each of the stepped frequencies and includepixel densities corresponding to the amplitude of said returned signaland the phase angle between said transmitted stepped, frequency signaland said returned signal.